Method of maintaining electeic cttbbents of constant fbequency



26,1929. w CADY Re. 17,245 METHOD OF MAINTAINING ELECTRIC CURRENTS 0F CONSTANT FREQUENCY Original Filed Ma'y 28, 1921 FIG.

FIG. 6.

INVENTOR I M405? 6. 640v Reiasued Mar. 26,1925. I

UNITED STATES PATENT ,OFFICE.

WALTER G. CADY, OF MIDDLETO'WN, CONNECTICUT, ASSIGNOR '10 RADIO CORPORA- TION OF AMERICA, CORPORATION 01 DELAWARE.

METHOD OF MAINTAINING ELECTRIC CURRENTSOF CONSTANT FREQUENCY.

origam no. 1,472,553,:1at i1 October so, 1923, Serial No. 473,434, tiled m 28, 1921. Application m V reissue filedlA'pril 8, 1925. Serial 80.21334.

Drvisiolv A.

The invention which forms the subject of mypresent application for Letters Patent is an improvement in the art of producing and maintaining alternatingcurrents of constant frequency. It is well known that, heretofore,

thedevelopment of such currents, to any very high degree of precision, has been unattainable by ordinary means and great dlfiiculty hasbeen experienced in producing alternating currents of high and constant frequency and free from fluctuations, due to dlsturbances in or near the generating system.

The useful applications of my invention arenun'lerous. It may be employed in the transmission or the reception of intelligence by means of high-frequency currents, or it may be used for the testing and measurementof such currents, in all cases where the frequency can be varied, and, in general, the invention is" applicable to currents of any frequency.

In an application filed by me on January 28, 1920, Serial No. 354,659, which has matured into Patent No. 1,450,246, April 3, 1923, l have shown and described what I have termeda piezo-electric resonator, which, in general, comprises a plate of piezo-electric crystal with coatings on its opposite faces. Such a device has a natural period of vibration, but when set in vibration by a source of alternating current connected to its coatings, the amplitude of such vibrations isvery slight, unless the frequency of the altearnatin current approximates or equals the naturdl frequency of the device, in which case the reaction of the crystal upon the circuit may be very great. In carrying out my present invention, I utilize this piezo-electrie resonator in the manner hereinafter to be described.

' The special properties of the piczo-clectric resonator that I take advantage of for my present purpose are-tirst: that property by virtue of which such a resonator, whose vibrations are maintained by impulses received from one electric circuit, may beused to transmit energy, in the form of an alternating current, into another circuit; second, that property which it possesses of modifying, by its reactions, the alternating current of a particular frequency or frequencies flowing to it; and third, the fact that the effective capacit of the resonator depends, in a manner whic will more fully hereinafter appear, upon the frequency of the current in the circuit with suitably prepared plate cut from a piezoelectric crystal, and provided with conduc- 'tive coatings, and utilizing the so-called transverse efiect; but all statements made apply equally to crystal preparations utilizmg the longitudinal effect, and, in general, to any" mechanical vibrating system whatever,

having suitable preparations of piezo-electric crystals for setting the system into vibration and for utilizing its reactions.

The nature and purpose of the invention may be most readilycomprehended'by reference to the diagrams which I haveused in its explanation and which are exhibited in the accompanying drawin g In this drawing l is a diagram of the Well known and universally recognized Armstrong-oscillating circuit, selected for illustrative urposes and showing my invention applied thereto; Fig. 2 is a d agram of a three-stage amplification system of wellknown type, with my invention applied thereto; Figs. 3 and 4 are other, diagrams similar to Fig. 1 illustrating further modifications; and Figs. 5 and 6 arediagrams used in explainingthe principles of the invention.

The above-described diagrams are illus- I .trative examples of various ways in whlch a piece-electric resonator may be employed to malntaln constant frequency in .an'oscillatory electric circuit.

they do 'not purport to show the only possible It Wlll be understood that arrangements for securin this result, and,

even as to those shown, it will be further understood'that the illustrated arrangements may be modified in various ways which, however, are obvious to those skilled in the art and familiar with high-frequency apparatus and which will not alter the essential part played by the piezo-electrie resonator.

I shall assume, for pur oses of this case, that, in each case, the hig sfrequency oscillations are produced through the agency of the three-electrode vacuum tube of the type commonly used in radio-telegraphy, but I may state that this is not essential and that the only requirement is that the source of energy shall be capable of generating currents of a frequency determined or controlled by the electric constants of the oscillating circuit, and when I use the term coatings, in.

referring to the resonator, I mean either thin layers of metal on the crystal itself, or metallic plates in fixed relation to the crystal, or, in general, any means whereby an electric charge may be conducted to the resonator in such a way as to produce an electric field in the proper direction through the crystal.

In order to make the mechanical vibrations as little damped as possible, I have departed from the practice of previous experimenters, of cementingtinfoil coatings directly on to the piezo-electric plate, and have placed the plate, entirely uncoated, between two or more plates of metal which are connected to the electric circuit and serve as the coatings. A small air space is left between the plate and the coatings. The crystal is free to vibratc in the space between the coatings.

With the above understanding, I now refer to Fig. 1. In this figure, the numeral 1 represents the filament of the vacuum tube,

2 is the filament battery, 3 a regulating resistance, 4 the grid and 5 the anode of the vacuum tube. The battery in the anode circuit is designated by 6. These are the main,

essential parts of one of the numerous types of circuit commonly used for the generation of high-frequencyoscillations, the other elements being the coils 7 and 8 1n the grid and I anode circuits, respectively, 9 a variable condenser in parallel with the coil 7, for the purpose of controlling the frequency of the oscillations, 10 the grid condenser, and 11 the leak. All'these are old and well known.

The plate or crystal of the piezo-electric resonator is indicated by 12 and this plate has four coatings 13, 14, 15 and 16, the two former being connected to the terminals of the coil 8, the two latter to the grid circuit, around the condenser 10.

The operation of the system is as follows: When the coupling between the two coils 7 and 8 is of proper character or value, the

circuit oscillates with a frequency dcternnned, in the main, by the capacity of condenser 9, and the self-inductance of the coil 7. If the capacity (if-condenser 9 bc varied, the frequency changes, and when a rate is reached corresponding to one of the natural modes of vibration of thepicZo-ehrrtric resonator, the. latter begins to vibrate, by rcason of the. electric field existing between the coatings 1,3 and 14. These vibrations,

through the piezo-electric action, cause electric chargesto be induced on the coatings l5 and 16, which, in turn, alter the potential difference across the condenser 10, and hence the potential of the id itself. I

' Whether the amp itude of the potential of the grid be thereby increased or diminished depends upon which of the coatings 15 or 16 is connected to the grid, and upon the phase is modified by the fact that the potential difference between the coatings 15-and 161s influenced by the periodic voltage already existing across the condenser 10. In order to effect the maximum reinforcement of the oscillations, it may be advisable to control the phase. of the vibrations of the resonator. This may be done, for example, by giving the coil 8 a certain resistance, or by inserting, in series with the coil 8, anotherinductance and a resistance. and connecting the coatings 13 and 14 in parallel with such inductance and resistance, instead of in parallel with the coil 8.

The frequency of the electric oscillations as determined by the inductance of coil 7 and the capacity of the condenser 9, may be that of the fundamental vibration of the piezo-electric resonator, or of one of its overtones. In any case, the two pairs of coatings on the resonator should have such a size and position, relatively to the ends of the crystal plate, as to cause the greatest possible amplitude of vibration at the desired freregions where the stresses in the plates are greatest when vibrating. For the funda mental,- t'or example, this is near the center of the plate. For the first overtone, which is twice the fundamental frequency. the plate vibrates in halves, with a node of pressure at the center; the pairs of coatings should therefore be positioned at points a quarter of the distance from each end of the plate. Either the fundamental or the first overtonc may be obtained. And so for other overtones. It is also of great importance that the coatings be so disposed, and the plate so supported that the vibrations shall be damped as little as possible. i

If, under the conditions assumed, while the oscillations are being reinforced, through the vibrations of the piczo-clcctric resonator, and

at any of the frequencies uentioncd above,

- capacity of condenser 10 very small. or even to omit it altogether. i

I have found that all of those factors that usually have a disturbing ellect upon the frequency, as, for example, variations in the voltage of either of the, batteries 2 or 6,

or movements of the hand near the circuit,

, are, by the use of the piezo-electric resonator, and within wide limits, without appreciable efi'ect upon the frequency.

The piezo-electric resonator may be used in connection with other types of oscillating circuit than the one above described. In accordance with the principle ofoperation set cuits.

The greater the amplification constant of the vacuum tube, the more widely' may the electrical constants of the circuit be altered, without affecting the frequency. For this reason, a still greater degree of stability may 'be attained by the use of a plurality of tubes,

connected for cascade amplification. I have found, for instance, that by the use of a cascade amplifier comprising three tubes, the coils 7 and 8 and the condenser 9 may be entirely dispensed with. i

, This arrangement of circuits is shown in Fig.2, in which 4 represents the grid of the first tube and the anode of the third. Two

condensers 19 and 20. This ilustrates the well known resistance amplification, but .any other type of amplifier might be em ployed. In this figure, 21 is a resistance or other impedance in the anode or output circuit, while the piezo-electric resonator 12 has its two coatings 13 and 14 connected to the plate 5, and the ground 22, respectively, and its two coatings 15 and 16 connected, the one to the ground 22, and the other to the grid 4.

The 0 eration of this arrangement is as follows: y'slight increase in the potential of the anode 5, by altering the electric field between the plates 13 and 14', sets the piezo-electric resonator in vibration. The charges thereb excited in the coatings 15 and -16 vary t e potential of the grid 4 with respect to the ground 22. This varying potential,

by virtue of the amplification taking place in the system, will, if the proper coating is connected with the grid 4, still further increase the variations in the potential ofthe anode 5 and maintain the ])l0Z()-(-!l(3tl'l0 reso-. nator 1n vibration. From the tcrmina-lsof the resistance or impedance 21, a small. amount of output power, at a constant frequency, may be drawn. a I i It is possible to secure constant frequency through the agency of a piezo-electric resonator which has only a single pair of plates or coatings, andthis may be accomplished, in various ways, by taking advantage of one or the other of the electricalefi'ects producedby the resonator when approximating one of its resonating frequencles. For instance, this in Fig.1, except that the piezo-electric resonator 12 has but one pair of coatings 13 and 14, which are connected in parallel with the condenser 10. If the capacity of the resonator be'suificiently large, the condenser may even be dispensed with. a

In explanation of the action which takes place in this case, let it be assumed that the coupling between the-coils 7 and 8 be loosened, as by increasing the distance between the two,

'until the circuit just fails, in the absence of the piezo-electric resonator 12, to oscillate.

When the resonator-is resent, however, and the. capacity of the con enser 9 is not too far from the value which would, with closer coupling, make the circuit oscillate at the same frequency as that of the vibrations of the resonator, the circuit will be found to be oscillating. The oscillations, in this case,

are due to the vibration of the resonator 12 and are of a frequency that is stable against disturbing influences, provided those latter be not too pronounced.

The theory of this regenerative action may be stated as follows: Suppose that, owing to some slight mechanical jar or electrical disturbance, the resonator is set into feeble vibrations. Through the piezo-electric action, these vibratlons cause corresponding changes in the potential of grid 4,'Fig. 3, which, in

.turn,-owing to the amplifying action of the vacuum tube and its associated circuits.-

causesimilar pulsating currents in the'coil 7.

These pulsating currents influence the charges on the coatings of the resonator and tend to maintain the vibrations of the latter.

Since the resonator is supposed to have been set into vibration by a somewhat sudden disturbance, it'follows, in accordance with well -known principles, that its vibrations, and

simply a single frequency, but may be con sidered as a combination of manyficquelicies of slightly different values, Of these different frequencies, that one predominates which is associated with maximum fluctuations of grid potential. I have shown, theoretically (Proceedings, Institute of Radio Engineers, vol. 10, p. 83, April, 1922), and verified by special experiments, that for the alternating electron'lotivc force acting in the circuit, the current flowing to the resonator is greatest .ata frequency very slightly below the natural frequency of said resonator. Also, when the resonator current is greatest, the cp'nsequent fluctuations in grid potential are naturally greatest. Hence, with the circuit shown-1n Fig. 3, an alternating current of practically unvary-ing frequency will be built up and maintained in the system,'of.a frequency slightly lower than the natural frequency of vibration of the resonator.

, plicability of] the portance in the example new to be considered.

Let it be assumed, in illustration, that the resonator is connected to a highefrequency, sinusoidal clectro-inotive force of constant voltage. lVhcn the frequency is considerably below that at which the resonator vibrates, the latter behaves like a simple condenser, having a certain capacity, which may be termed its normal capacity. As the frequency is raised to a value at which the piezoelectric resonator begins to vibrate perceptably, the elongation of the resonator plate is at first nearly in phase with the mechanical stress, which, in turn, is always exactly in phase with the voltage across the resonator.

Hence, in accordance with the well known laws of iezo-electricity, the piezo-electric polarization resulting from the elongation is nearly in phase with the impressed voltage,

' thus causing the total dielectric displacement in the piezo-electric than normal.

Since the apparent or, as it may be termed, the equivalent, capacity of the resonator is, other things being equal, proportional to the total dielectric displacement, it follows that so long ts the frequency is below the resonant value, the apparent capacity is abnormally large. It may, in fact, be many times in excess of the normal value. But after passresonator to be greater ing through a maximum, the apparent capacity decreases, and close to the resonant frequency, it returns to its normal value. Upon a further increase in frequency, the apparent capacity becomes abnormally small, in some cases even attaining a considerable negative value. If the frequency continues to increase, a HllllllIlIlIn-lll the apparent capacity is reached, after which 1t gradually returns to its normal value. These effects are due. to the change-1n phase of clfing'ittlOIlWll-ll respect to the driving force which the vibrating resonator, in common with all systems under-' going forced vibrations, experiences in the neighborhood of resonance.

I have represented these changes in the diagram-of Flg. 5, in which the apparentcapacity of the iezo-electric resonator, which may be denoted of fre uency, the apparent capacity has its norma value, represented by the point indiby C is plottedas a function r of the frequency f. Starting at alow value the tuning condenserfi as shown in Fig. l.

The diagram of this figure represents a vacuum-tube circuit, oscillating in the same manner as that shown in Fig. 1, through the nuitual induction of the coils 7 and 8, the frequency being determined bv the variable condenser 9. The coatings 13 and 14 of the reso- I nator 12 are in parallel with the condenser 9.

Let the capacity of the condenser 9 be denoted by C If the capacity C were constant,

. then a decrease in capacity C would result in a continuous increase in the frequency f, as

indicated in Fig. 6, by the curve 1, 9, 2, 4, 6,

8 and 7. Su )OSG, however and observin that the corresponding points in Figs. 5 and 6 are similarly numbered, that the piezoelectric resonator begins to vibrate perceptibly at a frequency corresponding to the point 2, thereupon the capacity C begins tobe abnormally large and therefore tends to diminish the frequency, with the result that the frequency increases less than it otherwise would. Continued decrease of the capacity C causes capacity C to increase still further, so that the curve bends down along-the portions 2, 3. At point 3,-the apparent capacity C has reached its greatest possible value, and if C be further decreased the resonator plate'suddenl ceases to vibrate, or, at best,' it vibrates-fee ly, at a much higher frequency, namely, the frequency corresponding to the point 8. In other Words, the curve springs abruptly fromthe point 3 to the point 8. From this on, any further decrease in C gives rise merely to the undisturbed portion 8, Tot the curve. If, on the other hand, the capacity C be increased, beginning at the point 7, the path 7, 8, .6, 5, 9, 1 will be traced, for reasons strictly analogous to those given for the curve describedon decreasing the capacity C, as stated above. The less damped the vibrations of the piezoelectric resonator, the more nearly constant will be the frequency over the portions of the curve 2, 3 and 5,6 in'Fig. 6. Hcnvc,.it is manifest that a resonator made from materials of good mechanical properties, such as quartz, or a combination of quartz and steel, suitably prepared and mounted, as has been set forth in my prior patent, will exert a marked stabiling effect upon the frequency of the generating circuit with which it is connected.

Any disturbing agent, as, for example, small changes in the value of C or in other capacities in the circuit, or variations in the filament current, will have. almost no effect at all upon the frequency, so long as such disturbing agent is not so pronounced as to cause the operating point on the curve to fall outside of the ranges 2, 3 and 5, 6.

Having now described my invention what .Iclaim is:

1. The combination with a generating systemof alternating current the frequency of which is capable of being determined by the varying potential difference betweentwo fixed points therein, of a piezo-electric resonator having two pairs of coatings, one pair being amplifier, the other pair connected to the input circuit so as to cause in the output circuit an alternating current to flow the frequency of which is determined by the mechanical vibrations of the said resonator.

3. An'oscillating system comprising two circuits and means for transferring energy from one of the circuits to the other circuit and a piezo-electric body connected with the system to feed back from the said other circuit .to the said one circuit, the piezo-electric body being designed to maintain the frequency of the oscillations substantially constant.

4. An oscillating system. comprising anoutput circuit, an input circuit having a condenser, and a piezo-electric body having a plurality of coatings connected around the condenser and with the output circuit, the body being designed to maintain the he ,quency of the oscillations substantially constant. v 5. An alternating-current system the fre-' quency of. which; is capable of being deter mined by the varying potential difference between two points thereof,the said system comprising a piezo-electric body having two 'pairs of coatings, the body being so designed and one of the pairs of coatings being so connected to the system'that the body shall be maintained ina state of vibration, and the other pair of coatings being connected with the said two points, whereby the frequencyof the oscillations will be maintained substantially constant. I Y

6. An alternating-current system comprising an input circuit, an output circuit and a piezo-electric body having two pairs of coatings, one of the pairs of coatings being connected with the output circuit, andthe other pair of coatings being connected with the input circuit, the body being so designed and connected that the frequency of the oscillations in the output circuit shall be substantially constant.

7. In an alternating-current system, a plurality of coatings connected withthe system,

and a piezo-electric body freely disposed betweenthe coatings, the body being so designed that its natural frequency of mechanical vibrations shall be in substantial resoname with a predetermined frequency of the current, whereby the body will react elec tri'cally upon the current at substantially the predetermined frequency. 8. In combination, a piezo-electric body and a plurality of coatings between which the body is freely disposed.

9. In an alternating-current system, a plurality of coatings connected with the system, and an electro-mechanical vibrator freely disposed -between the coatings, the vibratorbeing adapted to vibrate mechanically when stimulated electrically and to respond electrically when vibrated mechanically.

In testimony whereof, I hereunto afiix my signature. A Y

" WALTER (i. CADY. 

